add reduce with keys to collections

src/Illuminate/Collections/Collection.php

@@ -884,6 +884,24 @@ public function reduce(callable $callback, $initial = null)
         return array_reduce($this->items, $callback, $initial);
     }
 
+    /**
+     * Reduce an associative collection to a single value.
+     *
+     * @param  callable  $callback
+     * @param  mixed $initial
+     * @return mixed
+     */
+    public function reduceWithKeys(callable $callback, $initial = null)
+    {
+        $result = $initial;
+
+        foreach($this->items as $key => $value) {
+            $result = $callback($result, $value, $key);
+        }
+
+        return $result;
+    }
+
     /**
      * Replace the collection items with the given items.
      *